There are some wireless communication networks in which a part or all of communication areas (cells) covered by a plurality of base stations overlap each other. Such network is called a heterogeneous network.
In the heterogeneous network, in a communication area (macro cell) of one base station, communication areas (pico cells, femtocells) covered by one or more other base stations are deployed. In this specification, the former (the one base station) is called a main base station and the latter (the other base stations) are called subordinate base stations.
The main base station is a MN (master node). The MN is directly connected to a MME (Mobility Management Entity) (not shown). The MN can set one or more base stations as the subordinate base station (SN; second node) according to needs (for example, increase in data traffic). The SN is indirectly connected to the MME via the MN.
By using a DC (dual connectivity) technology, one or both of the MN and the SN can simultaneously provide a communication resource to one terminal.